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Publication Open Access Comparative spectroscopic investigation of Tm3+: tellurite glasses for 2-mu m lasing applications(Multidisciplinary Digital Publishing Institute (MDPI), 2018) Kurt, Adnan; Speghini, Adolfo; Bettinelli, Marco; Department of Electrical and Electronics Engineering; Department of Physics; Çankaya, Hüseyin; Görgülü, Adil Tolga; Sennaroğlu, Alphan; Researcher; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; College of Sciences; N/A; N/A; 23851We performed a comparative spectroscopic analysis on three novel Tm3+: tellurite-based glasses with the following compositions Tm2O3: TeO2-ZnO (TeZnTm), Tm2O3: TeO2-Nb2O5 (TeNbTm), and Tm3+: TeO2-K2O-Nb2O5 (TeNbKTm), primarily for 2-mu m laser applications. Tellurite glasses were prepared at different doping concentrations in order to investigate the effect of Tm3+ ion concentration as well as host composition on the stimulated emission cross sections and the luminescence quantum efficiencies. By performing Judd-Ofelt analysis, we determined the average radiative lifetimes of the H-3(4) level to be 2.55 +/- 0.07 ms, 2.76 +/- 0.03 ms and 2.57 +/- 0.20 ms for the TeZnTm, TeNbTm and TeNbKTm samples, respectively. We clearly observed the effect of the cross-relaxation, which becomes significant at higher Tm2O3 concentrations, leading to the quenching of 1460-nm emission and enhancement of 1860-nm emission. Furthermore, with increasing Tm2O3 concentrations, we observed a decrease in the fluorescence lifetimes as a result of the onset of non-radiative decay. For the H-3(4) level, the highest obtained quantum efficiency was 32% for the samples with the lowest Tm2O3 ion concentration. For the 1860-nm emission band, the average emission cross section was determined to measure around 6.33 +/- 0.34 x 10(-21) cm(2), revealing the potential of thulium-doped tellurite gain media for 2-mu m laser applications in bulk and fiber configurations.Publication Open Access Ultraslow optical modes in Bose-Einstein condensates(Society of Photo-optical Instrumentation Engineers (SPIE), 2007) Tarhan, Devrim; Postacıoğlu, Nazmi; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674Light can be slowed down to ultraslow speeds via electromagnetically induced transparency in atomic Bose-Einstein condensates. This is thought to be useful for storage of quantum information for weak probe pulses. We investigate the effects of inhomogeneous density profile of-the condensate on propagation of such ultraslow pulses. We find that spatial density of an atomic condensate leads to a graded refractive index profile, for an off-resonant probe pulse when condensate parameters are suitably chosen. Within the window of negligible absorption, conditions for degenerate multiple waveguide modes are determined. Both analytical and numerical studies are presented to reveal the effects of experimentally controllable parameters, such as temperature and interatomic interaction strength on the number of modes. Group velocity dispersion and modal dispersion are discussed. The effect of waveguide dispersion, in addition to usual material dispersion, on ultraslow pulses is pointed out.Publication Open Access Observation of two-photon interference using the zero-phonon-line emission of a single molecule(Institute of Physics (IOP) Publishing, 2006) Ehrl, M.; Hellerer, Th.; Brauchle, C.; Zumbusch, A.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Kiraz, Alper; Faculty Member; Faculty Member; Department of Physics; College of Sciences; 1674; 22542We report the results of coincidence counting experiments at the output of a Michelson interferometer using the zero-phonon-line emission of a single molecule at 1.4 K. Under continuous wave excitation, we observe the absence of coincidence counts as an indication of two-photon interference. This corresponds to the observation of Hong-Ou-Mandel correlations.Publication Open Access Dirac equation in spacetimes with non-metricity and Torsion(World Scientific Publishing, 2003) Adak, M.; Ryder, L.H.; Department of Physics; Dereli, Tekin; PhD Student; Faculty Member; Department of Physics; College of Sciences; 201358Dirac equation is written in a non-Riemannian spacetime with torsion and non-metricity by lifting the connection from the tangent bundle to the spinor bundle over spacetime. Foldy-Wouthuysen transformation of the Dirac equation in a Schwarzschild background spacetime. is considered. and it is shown that both the torsion and non-metricity couples to the momentum and spin of a massive, spinning particle. However, the effects are small to be observationally significant.Publication Open Access BCS theory of time-reversal-symmetric Hofstadter-Hubbard model(American Physical Society (APS), 2017) Umucalılar, Rıfat Onur; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659The competition between the length scales associated with the periodicity of a lattice potential and the cyclotron radius of a uniform magnetic field is known to have dramatic effects on the single-particle properties of a quantum particle, e.g., the fractal spectrum is known as the Hofstadter butterfly. Having this intricate competition in mind, we consider a two-component Fermi gas on a square optical lattice with opposite synthetic magnetic fields for the components, and study its effects on the many-body BCS-pairing phenomenon. By a careful addressing of the distinct superfluid transitions from the semimetal, quantum spin-Hall insulator, or normal phases, we explore the low-temperature phase diagrams of the model, displaying lobe structures that are reminiscent of the well-known Mott-insulator transitions of the Bose-Hubbard model.Publication Open Access Quantum state transfer among crystallographic groups of N-V centers in diamond(Society of Photo-optical Instrumentation Engineers (SPIE), 2011) Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We investigate collections of Nitrogen-Vacancy (N-V) Centers in diamond crystals coupled to a circuit QED system of a coplanar waveguide (CPWG) resonator. Our analysis reveals that different symmetry axes oriented N-V centers in the diamond host can be grouped into bosonic modes of collective quasi-spin wave excitations so that the hybrid system can be described as an analog of an exciton-polariton type cavity QED model. We examine such model for quantum state transfer among distinct crystallographic groups of N-V centers in a single diamond as well as two spatially distant diamonds. Rabi oscillations, mode entanglement, possible use of N-V classes as spin ensemble qubits and an implementation of continuous-time quantum random walk are discussed.Publication Open Access Topological superfluid phases of an atomic Fermi gas with in- and out-of-plane Zeeman fields and equal Rashba-Dresselhaus spin-orbit coupling(American Physical Society (APS), 2013) Subaşı, Ahmet Levent; Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659We analyze the effects of in-and out-of-plane Zeeman fields on the BCS-Bose-Einstein condensation (BEC) evolution of a Fermi gas with equal Rashba-Dresselhaus (ERD) spin-orbit coupling (SOC). We show that the ground state of the system involves gapless superfluid phases that can be distinguished with respect to the topology of the momentum-space regions with zero excitation energy. For the BCS-like uniform superfluid phases with zero center-of-mass momentum, the zeros may correspond to one or two doubly degenerate spheres, two or four spheres, two or four concave spheroids, or one or two doubly degenerate circles, depending on the combination of Zeeman fields and SOC. Such changes in the topology signal a quantum phase transition between distinct superfluid phases and leave their signatures on some thermodynamic quantities. We also analyze the possibility of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like nonuniform superfluid phases with finite center-of-mass momentum and obtain an even richer phase diagram.Publication Open Access Silicon photonics with microspheres(Society of Photo-optical Instrumentation Engineers (SPIE), 2006) Department of Physics; N/A; Serpengüzel, Ali; Akatlar, Onur; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; College of Sciences; College of Engineering; 27855; N/A; 194455Silicon microspheres coupled to optical fibers have been used for optical channel dropping in the near-IR. The observed morphology dependent resonances had quality factors of 100000. These optical resonances provide the necessary narrow linewidths, that are needed for high resolution optical filtering applications in the near-IR. In addition to filtering, detection, and switching applications of this photonic system is studied in the near-IR as well as far-IR. The silicon microsphere shows promise as a building block for silicon photonics in the near-IR as well as, mid-IR, and far-IR.Publication Open Access Dynamical properties of a coupled nonlinear dielectric waveguide-surface-plasmon system as another type of Josephson junction(Society of Photo-optical Instrumentation Engineers (SPIE), 2011) Department of Physics; Özok, Yasa Ekşioğlu; Müstecaplıoğlu, Özgür Esat; Güven, Kaan; Faculty Member; Faculty Member; Department of Physics; College of Sciences; N/A; 1674; 52290We demonstrate that a weakly-coupled nonlinear dielectric waveguide surface-plasmon (DWSP-JJ) system can be formulated in analogy to bosonic Josephson junction of atomic condensates at very low temperatures, yet it exhibits different dynamical features. Such a system can be realized along a metal - dielectric interface where the dielectric medium hosts a nonlinear waveguide (e. g. fiber) for soliton propagation. The inherently dynamic coupling parameter generates novel features in the phase space.Publication Open Access Ground-state cooling of mechanical resonatorsby quantum reservoir engineering(Springer Nature, 2021) Department of Physics; Müstecaplıoğlu, Özgür Esat; Naseem, Muhammad Tahir; Faculty Member; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 1674; N/ACooling a mechanical oscillator to its ground state underpins many applications ranging from ultra-precise sensing to quantum information processing. The authors propose a new scheme that addresses the problem of the simultaneous cooling of many mechanical resonators with nearby frequencies. Ground-state cooling of multiple mechanical resonators becomes vital to employ them in various applications ranging from ultra-precise sensing to quantum information processing. Here we propose a scheme for simultaneous cooling of multiple degenerate or near-degenerate mechanical resonators to their quantum ground-state, which is otherwise a challenging goal to achieve. As opposed to standard laser cooling schemes where coherence renders the motion of a resonator to its ground-state, we consider an incoherent thermal source to achieve the same aim. The underlying physical mechanism of cooling is explained by investigating a direct connection between the laser sideband cooling and ""cooling by heating"". Our advantageous scheme of cooling enabled by quantum reservoir engineering can be realized in various setups, employing parametric coupling of a cooling agent with the target systems. We also discuss using non-thermal baths to simulate ultra-high temperature thermal baths for cooling.